SPECK is a family of lightweight block ciphers proposed in 2013 by researches from National Security Agency(NSA) of USA. The algorithm adopts a modified Feistel construction that applies a combination of addition, rotation and XORing(the so-called ARX structure). Up to now, nothing is done on the impossible differential cryptanalysis of the SPECK family except that some 6-round impossible differential characteristics are found by LEE et al. In this article, some 6-round impossible differential characteristi...